BACKGROUND: NASA (July 2001) implemented an ergometer-based exercise prebreathe protocol (10 min at 75percent VO2 peak within a 120 min O2 prebreathe) for extra-vehicular activity (EVA) from the International Space Station. The current research involves exercising in the closed-circuit space suit to save oxygen and simplify infrastructure. This study evaluated the protocol for DCS protection and in-suit controllability. METHODS: A multi-center sequential trial (Duke/DRDC) was performed using accept criteria of DCS less than 15percent and Grade IV VGE less than 20percent at 95percent c.l., and reject criteria of DCS greater than 15percent and Grade IV VGE greater than 20percent at 70percent c.l. Forty-eight volunteers (38M/10F, 36.9+/-8.1 yrs, 44.0+/-7.6 [31.3-59.9] mL kg-1 min-1 VO2 peak) performed intermittent ergometer exercise (6 x 3 min exercise [at 60percent VO2 peak w/ 2 min rest] in a 120 minute protocol), depressurized to 4.3 psi (over 30 min), and performed 4 hrs simulated EVA work (approx 7.9 mL kg-1 min-1 ). Subjects were monitored for DCS and VGE. In a parallel study (NASA), 9 astronauts performed the same ergometer exercise. Oxygen consumption was measured with a metabolic cart (Quinton Qpex-I) and converted to tank pressure drop targets. The same astronauts exercised in the space suit, using O2-tank pressure feedback to control exercise intensity. Metabolic rates were measured with a mass spectrometer (Perkins-Elmer 1100A). RESULTS: Seven cases of Type I DCS (15percent) and 5 cases of Grade IV VGE (10percent) were observed in the decompression trials. DCS was significantly higher in subjects with VO2 peak less than 35 mL kg-1 min-1 (75percent) vs. VO2 peak greater than 35 mL kg-1 min-1 (9percent) (Fisher Exact p less than 0.01). NASA astronauts were able to control the in-suit exercise intensity to within an average +/-5.71percent compared to the ergometer. CONCLUSION: Neither an accept nor a reject decision was reached in the 50 trials. Control of in-suit exercise using O2-tank pressure drop is viable. Additional testing is planned. Funded by NASA and CSA.